An overview of Spire’s unique GNSS-R altimetry product is provided in this presentation.
Spire operates a constellation of smallsats equipped with an advanced GNSS receiver designed to collect radio occultation observations. In 2019, the receivers were configured to additionally measure GNSS reflections at grazing angles, i.e., between 5 and 30 degrees. Nguyen et al.  demonstrated the feasibility of phase delay altimetry using coherent reflections, following a number of prior studies (e.g., Martín-Neira , Cardellach et al. ). An in-house processing pipeline was subsequently established to generate Level 2 grazing angle altimetry products on an operational basis. As of November 2021, over 20 satellites continuously measure GNSS-R grazing angle reflection events in areas of high-coherence, i.e., calm waters and glaciated surfaces. Three satellites were added to the constellation in early November 2021.
Everyday, over 550 altimetry profiles are produced, spanning dozens to several thousand kms, without any inherent geographic limitation (i.e., no polar gap). Relative heights are provided with respect to a reference model. Over the oceans and sea ice, the large majority of profiles (>80%) can recover a reference surface model determined by a Mean Sea Surface and tides to within 50 centimeters. Bulk assessment of properties of altimetry profiles show that reflections occurring over sea ice are particularly long and coherent. Another opportune application of GNSS-R grazing angle altimetry pertains to land retrievals, for example in areas where reflections transition into glaciated land, such as the coastal regions in Antarctica. There, height retrievals over land surfaces have been shown to follow dynamic topographic changes and agree well with a reference DEM. Coherent retrievals are additionally available over higher altitude regions of the ice sheets, as well as glaciers and inland water bodies. We will also present the results of systematic assessments of more demanding altimetry applications such as sea ice freeboard height and sea level anomalies.
Given the wide array of potential scientific applications, the global and sustainable nature of Spire data collection, the availability of ancillary altimetry datasets and colocated measurements, and the unique measurement system (grazing angle GNSS-R from spaceborne platforms), there is a large body of work still to be accomplished. Studies by the geodesy community are encouraged and the data are freely available in agency portals such as through NASA CSDAP and ESA Earthnet.
Moreover, Spire data are not limited to Level 2 altimetry products. First, Level 1 products contain GNSS phase information alongside SNR and geometry information (position of transmitter, receiver, etc.). Complementary studies in the wider scientific community have already utilized such Spire reflection data to extract river heights and long-wavelength behavior of sea ice freeboard. Second, raw IF are also available upon request for dedicated studies over areas such as the Amazon, Great Lakes, etc. These types of lower level data are particularly useful to investigate components of the retrieval algorithm [Wang et al., 2020]. One example is phase unwrapping in high-noise environments, such as high elevation reflections (above 20 degrees) and/or rough sea state conditions. Furthermore, for low elevation reflections, an important source of error stems from the estimation of tropospheric delay, which has so far been mitigated via different approaches (different mapping functions, different atmospheric models, empirical corrections), with varying degrees of success.
Lastly, Spire satellites are continually undergoing modifications to improve and expand grazing angle observations. For example, multi-GNSS collection capability was recently implemented and tested on one satellite. One month of data collections showed that Galileo and GLONASS-based height retrievals were of the same quality and nearly as frequent and geographically diverse as those from GPS collections.
In conclusion, the Spire altimetry and reflections dataset presents a unique opportunity to 1) study the Earth’s surface and its evolution in a manner that is complementary to existing altimetry missions, 2) utilize the novel grazing angle dataset from space-based assets to research GNSS-R altimetry retrieval algorithms, and 3) harness a large volume of data (hundreds of thousands of altimetry profiles over oceans, sea ice, and land) to enable data mining in a diverse set of conditions.
Cardellach, E., Ao, C. O., De la Torre Juárez, M., & Hajj, G. A. (2004). Carrier phase delay altimetry with GPS-reflection/occultation interferometry from low Earth orbiters. Geophysical Research Letters, 31(10), L10402.
Martín-Neira, M., (1993), A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry, ESA J., vol. 17, no. 4, pp. 331–355.
Nguyen, V. A., et al. (2020), Initial GNSS Phase Altimetry Measurements From the Spire Satellite Constellation, Geophys. Res. Letters, vol. 47, no. 15.
Wang, Y., B. Breitsch and Y. T. J. Morton, "A State-Based Method to Simultaneously Reduce Cycle Slips and Noise in Coherent GNSS-R Phase Measurements From Open-Loop Tracking," in IEEE Transactions on Geoscience and Remote Sensing, vol. 59, no. 10, pp. 8873-8884, Oct. 2021, doi: 10.1109/TGRS.2020.3036031.